Harvey and Kirsten-murine sarcoma viruses have previously been shown to transform fibroblastic cells in culture and type C virus pseudotypes of these viruses cause erythroleukemia in susceptible mice. We have developed a cell culture assay for quantitating the growth-promoting effect of Harvey and Kirsten viruses on erythroid cells. Murine hemopoietic cells were infected in vitro with Harvey or Kirsten sarcoma virus and then cultured in methylcellulose. Under these conditions, large colonies of erythroid cells form in the semi-solid culture media 6-8 days post-infection (Erythroid Bursts). The induction of erythroid bursts was not caused by the murine type C helper viruses used to pseudotype either Ha-MSV or Ki-MSV or by media from cells carrying the Ki-MSV and Ha-MSV genomes. Induction of the erythroid colonies is under genetic control at the Fv-1 susceptibility locus, but not at the Fv-2 susceptibility locus. A striking feature of the Harvey and Kirsten virus induced erythroid colonies was that they not only proliferated to large size but also differentiated along the erythroid lineage and synthesized hemoglobin. It is concluded that Ha-MSV and Ki-MSV can induce proliferation of erythroid precursor cells without apparently interfering with the differentiation program of the cells.